Ultrasonic transducers for medical imaging have many components, and pitches among the components are getting smaller. As the dimensions of the components in an ultrasonic transducer decrease, a mismatch of electrical impedance between an ultrasonic image diagnostic system the ultrasonic transducer and an ultrasonic image diagnostic system is rising as a serious problem yet to be solved.
In general, electrical impedance of the components in an ultrasonic transducer ranges from 100 ohms to 500 ohms while electrical impedance of typical cables used to communicate between the ultrasonic transducer and the ultrasonic image diagnostic system ranges from 50 ohms to 85 ohms, exhibiting great difference therebetween. Such a mismatch deteriorates an energy transduction efficiency, which in turn results in a deterioration of sensitivity of the transducer and an increase of a signal-to-noise ratio, thereby impeding a signal processing for the representation of an ultrasonic image. The most important factors in ultrasonic image diagnosis are brightness and resolution of the image. However, the mismatch of the electrical impedance between the ultrasonic transducer and the ultrasonic image diagnostic system hinders the representation of a bright image.
In case piezoelectric substrates of same thicknesses are connected to each other in series acoustically but in parallel elastically, a relationship between voltage, impedance and the number of the piezoelectric substrates can be expressed as follows (see, Michael Greenstein and Umesh Kumar, “Multilayer piezoelectrical resonators for medical ultrasound transducer”, IEEE Transactions Ultrasonics, Ferroelectrics and Frequency Control, vol. 43, pp. 622-624, 1996):V(N)=V(1)/N Z(N)=Z(1)/N2 
where N, V, and Z represent the number of the wafers, voltage and impedance, respectively.
That is, as the number of the piezoelectric substrates increases, the impedance decreases in proportion to the square of N. Thus, by reducing the high impedance of the transducer's components based on this principle, it is likely that the above-mentioned mismatch problem would be solved.
In this regard, there have been made many attempts to apply a multilayer piezoelectric transducer to a medical ultrasonic transducer (see, David M. Mill et al., “Multi-layered PZT-Polymer Composites to increase signal to noise ratio and resolution for medial ultrasound transducer”, IEEE transactions on ultrasonics, ferro-electrics, and frequency control, Vol. 46, No. 4, July 1999).
Such a multilayer piezoelectric ultrasonic transducer as mentioned above, however, has a drawback in that it has a poor vibration feature because an additional layer besides a matching layer is coupled to a front surface of the transducer. For example, U.S. Pat. Nos. 6,121,718 and 6,437,487 disclose multiplayer ultrasonic transducers using piezoelectric materials, wherein FPCBs (Flexible Printed Circuit Boards) are formed on both the front and the rear surface of a stacked assembly for electrical connection. Therefore, the stacked assembly has a configuration in which a FPCB formed of a polyimide/Cu layer of several tens of microns or a Cu layer of several tens of microns is deposited on the front surface of the multilayer transducer. As a consequence, the vibration feature of the stacked assembly becomes poor.